Fuse protective circuit



Dec. 14, 1937. L. c. HART 2,102,204

FUSE PROTECTIVE CIRCUIT Filed June 20, 1956 INVEN TOE Patented Dec. 14,1937 UNITED STATES PATENT OFFICE FUSE PROTECTIVE CIRCUIT PennsylvaniaApplication June 2'0, 1936, Serial No. 86,252

1 Claim.

The invention relates to fuses and has for its principal objects theprovision of an improved fuse construction which will afford overloadprotection and short circuit protection. The ordi-. nary fuseconstruction employed for short circuit protection does not provide anyprotection against overload, because the ampere carrying capacity of thefuse is usually several times the normal full load current of theapparatus with which it is connected. Such fuses are ordinarily used toprotect the wiring system and service against a serious fault currentcaused by the apparatus connected to the system through the fuse, and inthis case, the apparatus would be damaged before the fuse would blow.When a fuse is used for overload protection, it is so proportioned tothe ampere capacity of the apparatus that the fuse will blow before theapparatus is damaged. In this case, the fuse will blow on a momentarycurrent increase which would in no Way damage the apparatus. The objectof the present invention is to provide both overload and short circuitprotection and obtain a time delay action for overload, so that theoverload fuse will not blow on a momentary increase in current, such asthat above referred to, the time delay being adjusted to suit thecharacteristics of the apparatus to be protected. In its preferred form,the assembly involves the use of two fuses in series, one proportionedto the overload and the other proportioned to the desired short circuitprotection, arranged so as to give a time delay action for the overload.Ilhe invention is illustrated lil the accompanying drawing, wherein:

Figure l is a diagrammatic view showing one arrangement of the device.Figs. 2 and 3 are detail views showing the fuse link assembly. Figs 4and 5 are views similar to that of Fig. 1 showing alternativearrangements. And Fig. 6 is a detail view showing a modification of theswitch construction.

Referring to Figs. 1, 2 and 3, the numerals 6 and I designate theterminals of a circuit in which the fuse construction is located; 8 and9 are the two fuse wires arranged in series in the circuit, 8 being theoverload protecting fuse and 9 the short circuit protecting fuse; and I0is a thermal switch arm in shunt with respect to the fuse 8 and makingcontact, when in the closed position shown in Figs. 1 and 3, with themember The overload fuse I is proportioned so that it will blow with acurrent load which is substantially lower than that it which the fuse 9will blow. It is designed to protect the apparatus connected with thecircuit, while the fuse 9 is designed to protect the wiring system frominjury in case of a short circuit or other accident throwing a heavyload on the circuit.

Under a normal current load, the switch III is 5 in closed position, butit is of the type which will open automatically upon a predeterminedincrease in load above normal. In the present instance, it is a thermalarm, well known in the art, consisting of two metals arranged so thatwhen the current load reaches a certain point, theheating of the metalscauses the arm to bend so that its end is separated from the member II.The switch is so designed that when a momentary increase in load occursin the circuit suflicient to blow the overload fuse 8 (if the switchwere not in shunt therewith) but not sufficient to injure the apparatusconnected to the system, 'the arm will heat up and open, thus causingthe full current flow to pass through the fuse 8. Since the 2 surge ofcurrent is only momentary, however, the flow is reduced by the time theswitch opens, so that the fuse 8 does not blow, and the necessity ofreplacing it is avoided. The switch thus gives a predetermined,appreciable time delay in exposing the fuse 8 to current surges. Athermal switch is suitable for giving this delayed action, but it willbe understood that other types of automatically operating switch deviceshaving a retarded opening might be employed.

One particular fuse link construction is shown in Figs. 2 and 3. As hereshown, the fuse wires 8 and 9 are carried by a bar of insulatingmaterial l2, such as Bakelite, the two adjacent ends being clampedtogether at one end of the bar by the member H in the form of a boltcarrying the nut l3 at one end and the head I4 at the other end. Thehead it is yieldingly engaged by the free end of the bi-metal switch armll under normal current conditions. The left hand ends of the switch armand the fuse 8 are clamped on opposite sides of the end of the bar l2 bythe bolt I5.

It will be apparent that in the operation of the construction justdescribed, the switch arm will close soon after it opens, since thecircuit is through the arm, and when the metals of the arm resumetheirnormal temperature due to the interruption of the current, the armwill return to its original position. In cases in which it is desired toavoid this automatic closure, a latch 50 maybe provided, as shown inFig. 6, to lock the arm in open position. The latch shown is a clip 4 I6of spring metal whose end II will move beneath the end of the arm whensuch end arrives at a point above the end IT.

Fig. 4 illustrates a modification of the circuit of Fig. 1 in which thethermal switch arm I8 is in the main circuit and is provided with an endI9 which normally bridges the gap between the con-' tact members 20 and2|, a wire 22 serving to connect the shunt circuit 23 carrying theoverload protecting fuse 8 with the arm l8. The operation parallels thatof the Fig. 1 circuit, as heretofore described.

Fig. 5 illustrates another arrangement, in which the fuses 8 and 9 arein parallel and in which 24 is the thermal switch arm. In case ofoverload, the switch opens throwing the current through the fuse 9, andthe device will function as

